Carbohydrate binding studies on the Bandeiraea simplicifolia I isolectins. Lectins which are mono-, di-, tri-, and tetravalent for N-acetyl-D-galactosamine.

Association constants for the binding of methyl alpha-D-galactopyranoside (methyl alpha-D-Galp) and methyl 2-acetamido-2-deoxy-alpha-D-galactopyranoside (methyl alpha-D-GalNAcp) to three Bandeiraea simplicifolia isolectins (A4, A2B2, B4) were determined by equilibrium dialysis and fluorescence enhancement measurements. The a and B subunits appear to have approximately the same Kassoc for methyl alpha-D-Galp: 1.45 X 10(4), 1.98 X 10(4), and 2.06 X 10(4) M-1 for A4, A2B2, and B4, respectively, as determined by equilibrium dialysis. Fluorescence enhancement measurements on B4 gave an association constant of 2.07 X 10(4) M-1 for methyl alpha-D-Galp and 1.87 X 10(3) M-1 for methyl beta-D-galp. By equilibrium dialysis, we were able to detect 3.3 (theory, 4.0) methyl alpha-D-GalNAcp binding sites for A4 (Kassoc = 1.87 X 10(5) M-1), 1.9 for A2B2 (Kassoc = 1.19 X 10(5) M-1), and were unable to detect any methyl alpha-D-GalNAcp binding sites for B4. However, four very weak methyl alpha-D-GalNAc binding sites for B4 were detected by fluorescence enhancement measurement (Kassoc = 1.26 X 10(2) M-1). Thus, the A subunit has an affinity for methyl alpha-D-GalNAc 3 orders of magnitude greater than the B subunit. Precipitation and hapten inhibition data are in accord with these binding measurements. Toward guaran and type B blood group substance, all isolectins precipitated the same amount of biopolymer. However, AB3, A2B2, and A3B, which are mono-, di-, and trivalent for alpha-D-GalNAcp, were differentially precipitated by type A blood group substance which contains alpha-D-GalNAcp-end groups. A3B precipitated the most, A2B2 less, and AB3 no type A substance. These isolectins should prove useful in studies evaluating the effect of valence on lectin-cell interaction.